GB382438A - Improvements in or relating to acoustic filters or silencers - Google Patents

Abstract

382,438. Exhaust silencers ; acoustic filters. ELECTRICAL RESEARCH PRODUCTS, Inc., 195, Broadway, New York, U.S.A., and KEITH, C. R., 4, Arcade House, Finchley Road, London. Nov. 17, 1931, No. 31857. [Class 7 (ii).] An acoustic filter or silencer consists of an elongated conduit having a longitudinal slit or a number of lateral openings along its length, the conduit and the slit or openings being relatively so proportioned that a pressure impulse imparted to a gaseous stream in the conduit causes the expulsion of substantially equal volumes of the gaseous medium through successive equal lengths of the slit or through successive lateral openings in the conduit. The Specification contains expressions for the sizes of the holes, or the width of the slit, and gives a numerical example for a pipe one inch in diameter with fifty holes adapted to eliminate a fundamental frequency of sixty cycles per second in an engine running at 1800 revolutions per minute. The conduit may be of uniform diameter throughout with the perforations or slit increasing in area towards the outlet end, or the conduit may decrease in area towards the outlet end with perforations or slit of uniform area throughout. The conduit 10, as shown in Fig. 1, may be in the form of a helix, the perforations being shown at 11. It is supported by brackets 15 within a casing 13 into which the outlet pipe 14 projects some distance. In a modification, the conduit is of flat spiral form in a box-like casing ; it decreases in cross section towards the outlet and has a number of holes of uniform diameter spaced equally throughout its length.